In-line cell for absorption spectroscopy

ABSTRACT

Provided is a novel in-line cell useful in absorption spectroscopy. The cell includes a sample region, a light entry port and a light exit port being the same or separate ports. Each port is in communication with the sample region and contains a light transmissive window. A mirror having a light reflective surface faces the sample region, and a heater effective to heat the light reflective surface is provided. The cell can be used to determine the concentration of molecular gas impurities in a sample. Particular applicability is found in semiconductor processing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of application Ser. No.08/711,504 filed Sep. 10, 1996 and now U.S. Pat. No. 5,818,578, whichapplication is a Continuation-In-Part of application Ser. No. 08/634,436filed Apr. 18, 1996, abandoned. The contents of those applications areherein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel in-line cell useful inabsorption spectroscopy. The present invention also relates to a systemfor performing an absorption spectroscopy measurement in a sample and toa semiconductor processing apparatus which comprise the novel in-linecell.

2. Description of the Related Art

Semiconductor integrated circuits (ICs) are manufactured by a series ofprocesses, many of which involve the use of gaseous materials. Includedamong such processes are etching, diffusion, chemical vapor deposition(CVD), ion implantation, sputtering and rapid thermal processing.

Many of the gases used in these processes are highly reactive and tendto form deposits on surfaces with which they come into contact. When anin-line spectroscopic sensor is used to monitor a process in suchaggressive atmospheres, deposits from the process gases tend to form onvarious surfaces of the sensor. As a result, sensor performance tends todeteriorate.

The sensitivity of detection of gas phase molecular species byabsorption spectroscopy increases as the length of the light paththrough the sample increases, for constant pressure and concentration.The intensity of light reaching the detector is given by Beer's Law asfollows:

    I=I.sub.o ·e.sup.-αlcP

where I_(o) is the intensity of the incident radiation, α is theabsorptivity, l is the pathlength through the sample, c is theconcentration of the impurity in the sample (by volume), and P is thetotal pressure of the sample. For small absorptions, the amount of lightabsorbed is given by

    I-I.sub.o =αlcP

In order to make l large, it is frequently impractical to place thelight source and detector very far apart and so "folded" light paths areoften used, in which mirrors reflect the light back and forth throughthe sample gas many times.

The Herriott design is often preferred for tunable diode laserabsorption spectroscopy (TDLAS). As shown in FIG. 1, the Herriott cell100 uses two curved mirrors 102 mounted at opposite ends of a usuallycylindrical gas sample cell 104. Simple multi-pass arrangements areoften used, such as described in U.S. Pat. No. 3,524,066, to Blakkan,and U.S. Pat. No. 5,173,749, to Tell et al, the contents of which areherein incorporated by reference. A planar polygonal multipass cell isdescribed by the present inventors in copending application Ser. No.08/711,504, filed Sep. 10, 1996, the contents of which are hereinincorporated by reference.

In the multipass cells described above, deposits formed on thereflective surfaces of the mirrors can reduce their reflectivity andhence the light intensity which reaches the detector after multiplereflections of the light beam. This reduction in light intensity reducesthe measurement sensitivity and may eventually lead to a condition inwhich the sensor does not function at all.

Deposits on the mirrors can be removed by disassembling the sensor andmechanically cleaning the mirror(s). Such maintenance, however, isinconvenient and expensive. Avoidance thereof is, therefore, desirable.

To meet the requirements of the semiconductor processing industry and toovercome the disadvantages of the related art, it is an object of thepresent invention to provide a novel in-line cell useful in absorptionspectroscopy. The in-line cell allows for accurate, in-situ absorptionspectroscopy measurements which can be used, for example, to accuratelyand sensitively measure the concentration of gas phase molecularimpurities in a sample. The problems associated with the formation ofdeposits on reflective surfaces of mirrors within the measurement cellare avoided or conspicuously ameliorated by the inventive cell.

It is a further object of the present invention to provide an absorptionspectroscopy system which includes the inventive in-line cell.

It is further an object of the present invention to provide asemiconductor processing apparatus which includes the absorptionspectroscopy system for performing in-situ measurements.

Other objects and aspects of the present invention will become apparentto one of ordinary skill in the art on a review of the specification,drawings and claims appended hereto.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, a novel in-linecell useful in absorption spectroscopy is provided. The in-line cellincludes a sample region, a light entry port and a light exit port. Thelight entry port and light exit port can be the same or separate ports.Each port is in communication with the sample region and contains alight transmissive window. A mirror having a light reflective surfacefaces the sample region, and a heater effective to heat the lightreflective surface is provided.

The in-line cell allows for accurate, in-situ absorption spectroscopymeasurements which are useful, for example, to accurately andsensitively measure the concentration of gas phase molecular impurities,such as, e.g., methane, moisture (water vapor) and carbon dioxide, in asample. In particular, the light reflective surfaces of the cell can bemaintained in a deposit-free state.

According to a further aspect of the invention, a system for performingan absorption spectroscopy measurement is provided. The system includesan in-line cell as described above with reference to the first aspect ofthe invention. The inventive system further comprises a light source forgenerating a light beam which passes through the light entry port intothe cell, and a main detector for measuring the light beam exiting thecell through the light exit port.

According to a third aspect of the invention, a semiconductor processingapparatus is provided. The apparatus comprises a vacuum chamber incommunication with a vacuum pump for evacuating the vacuum chamber, andthe inventive absorption spectroscopy measurement system describedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the invention will become apparent fromthe following detailed description of the preferred embodiments thereofin connection with the accompanying drawings, in which:

FIG. 1 is a conventional absorption spectroscopy cell according to theHerriott design;

FIGS. 2A and 2B are top and side-sectional views of an in-line cellaccording to one aspect of the present invention;

FIG. 3 is a top view of a light source/detector scheme in a system forperforming an absorption spectroscopy measurement according to oneaspect of the present invention; and

FIG. 4 is a side sectional view of a semiconductor processing apparatuswhich includes a system for performing an absorption spectroscopymeasurement in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 2A illustrates a top sectional view of an exemplary in-line celluseful in absorption spectroscopy according to one aspect of theinvention, and FIG. 2B shows a side-sectional view taken along line A--Aof FIG. 2A. While the exemplary cell is of the Herriott type ofmultipass cell, it should be clear that the inventive concepts describedhereinbelow are in no way limited thereto, and can readily be applied toother forms of cells.

The in-line cell 200 includes a sample region 202, which is bounded bysidewalls 204, top wall 206 and bottom wall 208. Mirrors 210 aredisposed at opposite ends of the cell, and have light reflectivesurfaces facing the sample region. The light reflective surfaces arepreferably a polished metal. As a high reflectivity of these surfaces isdesirable, the surfaces can be coated with one or more layers of areflective material such as gold, other metallic layers or a highlyreflective dielectric coating, in order to enhance the reflectivitythereof.

The cell further includes a light entry and exit port 212 for allowing alight beam to pass into the cell and to pass out of the cell. While theexemplary embodiment shows a single port through which the light beamenters and exits the cell, a plural port structure is also envisioned.Thus, the light beam can enter and exit the cell through the same ordifferent ports in the cell, and can enter and/or exit the cell throughplural light entry or light exit ports. Further, the ports can bedisposed on the same side or different sides of the cell.

The entry/exit port 212 contains a light transmissive window 214 whichallows the light beam to pass into and out of the sample region.Suitable light transmissive materials for the window are known andinclude, for example, aluminum oxide, quartz and magnesium fluoride. Themirrors 210 and light transmissive window 214 seal the cell in asubstantially airtight manner. This makes possible the measurement ofgas samples at low pressures, i.e., at vacuum conditions, therebyallowing for in-situ measurements in vacuum processing tools such asused in the semiconductor manufacturing industry.

As shown in the exemplified cell, light entry and exit port 212 andlight transmissive window 214 can be incorporated into the mirror. Withsuch a structure, the size of the cell can be minimized. Of course, awindow which is distinct from the entry and/or exit port can beprovided.

Light transmissive window 214 can additionally be provided with acoating layer on a surface opposite the surface facing the sample regionfor reflecting a portion of the light beam. Subtracting the signal dueto the reflected portion of the beam from that of the transmittedportion can result in more accurate absorption measurements thanotherwise possible. Among the commercially available coating materials,metallic coatings are preferred.

As shown in the exemplified embodiment, light transmissive window 214can be offset at an angle from perpendicular relative to the incominglight beam, with the Brewster angle being preferred. By offsetting thewindow in this manner, coherent interference caused by the reflectedportion of the beam can be avoided. As a result, measurement of greateraccuracy can be obtained.

Such an offset can further serve the same purpose as that served by theoptional coating layer described above. That is, the reflected portionof the beam can be used to obtain a more accurate measurement bysubtracting out the signal due to background noise.

The exemplified in-line cell further includes a sample inlet port 216and flange 218 for connection to a source for the sample to be measured.Inlet port 216 can advantageously be connected to an exhaust line from asemiconductor processing tool in order to perform in-situ measurements,for example, to determine concentrations of molecular gas species in thetool exhaust.

The gas sample passes through the cell from inlet port 216 into sampleregion 202 and out of the cell through exhaust port 218. The exhaustport can be connected to a suitable vacuum pump. The connection with thepump can be made using flange 220 and flexible hose 222.

To minimize the adverse effects an absorption signal caused by depositsformed on the light reflective surfaces, one or more heaters 224 forheating the light reflective surfaces of the mirrors are provided. Theheater should be capable of raising the mirror's light reflectivesurface to a temperature of from about 50 to 150° C., preferably fromabout 70 to 100° C., although optimal temperatures are processdependent.

Suitable heaters include, but are not limited to, resistance-typeheaters, self-regulating-type heaters such as heat trace, heating lamps,inductive heaters and liquid or gaseous heating fluids which optionallycan be circulated.

The cell is constructed in such a fashion that the mirrors can bemaintained at a temperature higher than that of nearby surfaces in thecell which are exposed to the atmosphere to be analyzed. Since thedeposits tend to form on lower temperature surfaces, deposits can beeffectively prevented from forming on the heated mirror surface facingthe sample region.

Heating of the mirrors as described above should be contrasted with thesituation in which the entire cell is heated. In such a case, heattransfer from the body of the cell to the mirrors is not facilitated.Consequently, the mirrors would tend to be at a lower temperature thanthe walls of the cell, resulting in deposits being concentrated on themirrors.

A particularly advantageous way to achieve the objectives of theinvention is by the integration of the one or more mirrors into one ormore walls of the cell, with the heater(s) being disposed outside of thecell. This allows heat to be directly applied to the back-surface of themirror. In this manner, it is possible to attain the objective ofmaintaining the mirror surface at a higher temperature than othersurfaces in the cell, while at the same time, isolating the heater fromthe aggressive environment in the sample region. This is especiallydesirable since placement of the heater directly in the sample regionwould result in corrosion or other damage to the heater.

In the exemplified cell, mirrors 210 are integrated into the end flangesof the cell. The mirrors can be fixed in place by known methods, such aswelding, to ensure a substantially air-tight seal. Integration of themirrors into the walls of the cell is not, however, limited to theexemplified method.

The mirror itself, or any mirror mount which provides sufficient heatconduction and makes good thermal contact with the mirror, can bebrought into direct physical (and thermal) contact with a heatingelement or, as in the case with lamp-type heaters, into direct thermalcontact with the heating element located outside the cell. Generally, aslong as the thermal resistance of the path from the heater to the mirrorsurface is less than the thermal resistance from the heat path to othersurfaces inside the sample cell (or any nearby region), then the desiredresults can be achieved.

In addition to the above-described mirror heaters, a purge gas flow canoptionally be introduced into the cell to further prevent theaccumulation of deposits on the light reflective surfaces. Suitablepurge gases include an inert gas, such as nitrogen, argon or helium. Ifthe purge gas is heated, it can serve the purpose of the mirror heaterdescribed above. Preferably, such heated gas is localized on the lightreflective surface. Therefore, such a heated purge gas stream can beused by itself or together with other types of heaters such as describedabove, to effectively prevent deposits on the light reflector surfaces.For an unheated purge gas stream, it is preferable to localize the flowof the gas in the vicinity of the light reflective surface. Byminimizing flow of the purge gas, the need for retrofitting vacuumsystems with larger capacity pumps will be eliminated.

In addition to the heating of the mirrors, the light transmissivewindows can optionally be heated. In the exemplified cell, lighttransmissive window 214 can be heated using the same heater or a heaterindependent of that used to heat the mirror. Furthermore, a purge gasstream as described above with reference to the light reflectivesurface, can also be applied to the light transmissive window. By use ofsuch a structure, deposits can be further prevented from forming on thewindows.

Although the exemplified in-line cell is of a Herriott design, theinventive concepts can readily be applied to other types of in-linecells. For example, the inventive concepts can be applied to polygonalmultipass cells or to any form of cell which is amenable to the aboveconditions with respect to the mirror and heater structures.

The in-line cell should be constructed of materials which are compatiblewith the atmospheres being contained therein. Such materials are withinthe knowledge of persons skilled in the art. For example, various formsof stainless steel can be used for cell surfaces which contact thesample being measured.

While the inventive cell can be used for any absorption spectroscopytechnique, it is preferably used in tunable diode laser absorptionspectroscopy (TDLAS). With reference to FIG. 3, such a system includes,in addition to the in-line cell as described above with reference toFIGS. 2A and 2B, a light source 302, preferably a diode laser, forgenerating a light beam which is directed through the light transmissivewindow 304 into the sample region of the cell. To measure the light beamwhich exits the cell through the light transmissive window, the systemfurther includes a main detector 306, which can be, for example, aphotodiode.

Any molecular impurity of interest can be detected, subject only to theavailability of a suitable light source. For example, water vapor,nitric oxide, carbon monoxide and methane or other hydrocarbons can bedetected by measuring the attenuation of light from a diode laser sourcewhich emits light of a wavelength characteristic of the impurity.

Laser light sources which emit light in spectral regions where themolecules of interest absorb most strongly lead to improvements inmeasurement sensitivity. In particular, light sources which emit atwavelengths longer than about 2 μm are preferred, since many of themolecular impurities of interest have strong absorption bands in thisregion.

Any suitable wavelength-tunable light source can be used. Of thecurrently available light sources, diode laser light sources arepreferred because of their narrow linewidth (less than about 10⁻³ cm⁻¹)and relatively high intensity (about 0.1 to several milliwatts) at theemission wavelength.

Examples of diode lasers include Pb-salt and GaAs-type diode lasers. ThePb-salt-type laser requires cryogenic temperatures for operation andemits infrared light (i.e., wavelength greater than 3 μm), while theGaAs-type diode laser can operate at close to room temperature and emitsin the near infrared region (0.8-2 μm).

Recently, diode lasers which include Sb in addition to GaAs (or otherpairs of III-V compounds such as AsP) have been described (see,"Mid-infrared wavelengths enhance trace gas sensing," R. Martinelli,Laser Focus World, March 1996, p. 77). These diodes emit light of awavelength greater than 2 μm while operating at -87.8° C. While such alow temperature is not convenient, it compares favorably with thecryogenic temperatures (less than -170° C.) required by Pb-salt lasers.

Operation of similar lasers at 4 μm and 12° C. has also been reported(see, Lasers and Optronics, March 1996). Diode lasers of the abovedescribed type will most preferably operate at temperatures of at least-40° C. Use of a thermoelectric cooler for temperature control at suchtemperatures makes these light sources less complicated than the lowertemperature diode systems.

To make use of these lasers more desirable, improvement in the opticalproperties over current levels is important. For example, single modediodes (i.e., diodes whose emission at fixed temperature and drivecurrent is at a single wavelength with emission at other wavelengths atleast 40 dB less intense) should be available.

Suitable light sources for use in the invention are not limited to theabove described diode lasers. For example, other types of lasers whichare similarly sized and tunable by simple electrical means, such asfiber lasers and quantum cascade lasers, are envisioned. The use of suchlasers as they become commercially available is envisioned.

The system can further include at least one mirror 308 for reflectingthe light beam 310 from the light source 302 through the lighttransmissive window into the cell, and at least one additional mirror312, 314 for reflecting the light beam exiting the cell to the maindetector.

The mirror 308 is preferably curved in order to collimate the light beamas the light from the diode laser source is divergent. Likewise, mirror314 is preferably curved in order to focus the parallel light beam onthe main detector.

A second detector 316, which can also be a photodiode, for measuring aportion of the light beam 318 which is reflected from the lighttransmissive window 304 as well as means for subtracting this referencesignal from a measurement obtained by the main detector can optionallybe provided in the system. An operational amplifier in a configurationsuch as described in the literature (See, e.g., Moore, J. H. et al"Building Scientific Apparatus", Addison Wesley, London, 1983) can actas the means for subtracting the reference signal.

The reflected light does not show any absorption by the molecules ofinterest in the sample region, and therefore provides a referencesignal. By subtracting the reference signal from that of the light whichpasses through the cell (which is measured by the main detector),variations in the light source can be compensated for. This also allowsfor enhanced sensitivity to signal changes due to molecules in thesystem chamber 320.

While "dual beam" techniques using subtraction of a reference beam arewell-known they usually require a dedicated beam-splitter, i.e., anoptical element whose only function is to divide the light beam.According to the present invention, the entrance window to the chambercan provide this function without the need for any additionalcomponents. The ratio of transmitted to reflected light at this windowcan be controlled by use of an appropriate coating for the window.

The inventive system has particular applicability in detecting amolecular species in a gas exhausted from a vacuum chamber. In such acase, the cell can be disposed in a vacuum exhaust line between a vacuumchamber and a vacuum pump system.

The system is compatible with a wide range of materials. For example,the vacuum chamber can contain certain reactive or nonreactive (inert)gas species which can be in a plasma- or non-plasma state. Examples ofreactive gases which are compatible with the inventive system includeSiH₄, HCl and Cl₂ provided the moisture level is less than 1000 ppm. Anyinert gas such as, e.g., O₂, N₂, Ar and H₂ can be used in the inventivesystem. In the case of the inventive system's use in a plasmaenvironment, the system is preferably mounted about 6 inches or moreaway from the plasma zone in order to minimize the formation of depositson the windows and other cell surfaces.

Because the detection system described above can be used in plasma ornon-plasma atmospheres as well as with inert or reactive gases, thesystem is particularly well suited for use in monitoring gas phasemolecular species, such as water vapor, in a semiconductor processingapparatus. Use of the detection system in conjunction with asemiconductor processing apparatus allows for real time in-situmonitoring of gas phase molecular impurities.

The system can be readily adapted to virtually any semiconductorprocessing apparatus which employs a vacuum system. Examples of suchapparatuses include etching, diffusion, chemical vapor deposition (CVD),ion implantation, sputtering and rapid thermal processing apparatuses.

FIG. 4 illustrates a semiconductor processing system 400 which comprisesan in-line cell and system 401 for performing absorption spectroscopymeasurements as described in detail above. The system further includes avacuum chamber 402 inside which a semiconductor substrate 404 isdisposed on a substrate holder 406. One or more gas inlets 408 areprovided for delivering a gas or plural gases to the vacuum chamber.

The vacuum chamber is evacuated through an exhaust opening 410 in thevacuum chamber. A portion of the total exhaust from the processing toolor the entire exhaust volume can be introduced into cell 411. A vacuumpump 412 for evacuating the vacuum chamber is connected thereto, eitherdirectly or through a vacuum line. A pump exhaust line 414 can beconnected to the pump 412, which can be connected to another pump or toa gas scrubber (not shown). Examples of vacuum pumps which may beemployed are mechanical rotary and booster pumps, diffusion pumps,cryogenic pumps, sorption pumps and turbomolecular pumps.

Furthermore, while the vacuum pump and measurement system have beenillustrated as being disposed below the vacuum chamber, those skilled inthe art readily understand that other orientations are also possible.

While the invention has been described in detail with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made, and equivalentsemployed, without departing from the scope of the appended claims.

What is claimed is:
 1. An in-line cell useful in absorptionspectroscopy, comprising a sample region, a light entry port incommunication with the sample region to allow a light beam to enter thecell and pass through the sample region, a light exit port incommunication with the sample region to allow a light beam after passingthrough the sample region to exit the cell, the light entry port andlight exit port being the same or separate ports, each said portcontaining a light transmissive window, a mirror having a lightreflective surface facing the sample region for reflecting a light beamwithin the cell, and a heater effective to heat the light reflectivesurface of the mirror.
 2. The in-line cell according to claim 1, whereinthe heater is disposed outside of the cell.
 3. The in-line cellaccording to claim 1, wherein the heater is in direct contact with aback surface of the mirror.
 4. The in-line cell according to claim 1,wherein the thermal resistance of a path from the heater to the mirroris less than the thermal resistance of a path from the heater to othersurfaces of the sample cell.
 5. The in-line cell according to claim 1,wherein the heater is a resistance-type heater.
 6. The in-line cellaccording to claim 1, wherein the cell is a multipass cell.
 7. Thein-line cell according to claim 1, further comprising a purge gas inleteffective to introduce a purge gas stream which contacts the mirror. 8.The in-line cell according to claim 1, further comprising a heater forheating each light transmissive window.
 9. The in-line cell according toclaim 1, wherein the light transmissive window in the light entry portis offset from perpendicular with respect to an incoming light beam. 10.The in-line cell according to claim 1, wherein the light reflectivesurface is integrated into a wall of the cell.
 11. A system forperforming an absorption spectroscopy measurement, comprising:a sampleregion, a light entry port in communication with the sample region toallow a light beam to enter the cell and pass through the sample region,a light exit port in communication with the sample region to allow alight beam after passing through the sample region to exit the cell, thelight entry port and light exit port being the same or separate ports,each said port containing a light transmissive window, a mirror having alight reflective surface facing the sample region for reflecting a lightbeam within the cell, and a heater effective to heat the lightreflective surface of the mirror, and a light source for generating alight beam which passes through the light entry port into the cell, anda main detector for measuring the light beam exiting the cell throughthe light exit port.
 12. The system according to claim 11, wherein theheater is disposed outside of the cell.
 13. The system according toclaim 11, wherein the heater is in direct contact with a back surface ofthe mirror.
 14. The system according to claim 11, wherein the thermalresistance of a path from the heater to the mirror is less than thethermal resistance of a path from the heater to other surfaces of thesample cell.
 15. The system according to claim 11, wherein the heater isselected from the group consisting of resistance-type heaters,self-regulating-type heaters, heating lamps, inductive heaters and aheating fluid.
 16. The system according to claim 11, further comprisinga purge gas inlet effective to introduce a purge gas stream whichcontacts the mirror.
 17. The system according to claim 11, furthercomprising a heater for heating each light transmissive window.
 18. Thein-line cell according to claim 11, wherein the light transmissivewindow in the light entry port is offset with respect to an incominglight beam from perpendicular.
 19. The system according to claim 11,wherein the cell is disposed between and in communication with a vacuumchamber and a vacuum pump.
 20. A semiconductor processing apparatus,comprising:a vacuum chamber in communication with a vacuum pump forevacuating the vacuum chamber, an in-line cell disposed between and incommunication with the vacuum chamber and the vacuum pump, the cellcomprising a sample region, a light entry port and a light exit portbeing the same or separate ports, each said port being in communicationwith the sample region and containing a light transmissive window, amirror having a light reflective surface facing the sample region, and aheater effective to heat the light reflective surface of the mirror alight source for directing a light beam through the light entry portinto the cell, and a main detector for measuring the light beam exitingthe cell through the light exit port.